On Fri, 14 Jun 2024, Seyed Sajad Kahani wrote:
> When deducing auto for `adc_return_type`, `adc_variable_type`, and
> `adc_decomp_type` contexts (at the usage time), we try to resolve the
> outermost
> template arguments to be used for satisfaction. This is done by one of the
> following, depending on the scope:
>
> 1. Checking the `DECL_TEMPLATE_INFO` of the current function scope and
> extracting DECL_TI_ARGS from it for function scope deductions (pt.cc:31236).
> 2. Checking the `DECL_TEMPLATE_INFO` of the declaration (alongside with other
> conditions) for non-function scope variable declaration deductions
> (decl.cc:8527).
>
> Then, we do not retrieve the deeper layers of the template arguments; instead,
> we fill the missing levels with dummy levels (pt.cc:31260).
>
> The problem (that is shown in PR114915) is that we do not consider the case
> where the deduction happens in a template specialization scope. In this case,
> the type is not dependent on the outermost template arguments (which are
> the specialization arguments). Yet, we still resolve the outermost template
> arguments, and then the number of layers in the template arguments exceeds the
> number of levels in the type. This causes the missing levels to be negative.
> This leads to the rejection of valid code and ICEs (like segfault) in the
> release mode. In the debug mode, it is possible to show as an assertion
> failure
> (when creating a tree_vec with a negative size).
>
> This patch resolves PR114915 by replacing the logic that fills in the
> missing levels in do_auto_deduction in cp/pt.cc.
> The new approach now trims targs if the depth of targs is deeper than desired
> (this will only happen in specific contexts), and still fills targs with empty
> layers if it has fewer depths than expected.
LGTM
>
> PR c++/114915
>
> gcc/cp/ChangeLog:
>
> * pt.cc (do_auto_deduction): Handle excess outer template
> arguments during constrained auto satisfaction.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp2a/concepts-placeholder14.C: New test.
> * g++.dg/cpp2a/concepts-placeholder15.C: New test.
> * g++.dg/cpp2a/concepts-placeholder16.C: New test.
> ---
> gcc/cp/pt.cc | 20 ++++++++---
> .../g++.dg/cpp2a/concepts-placeholder14.C | 19 +++++++++++
> .../g++.dg/cpp2a/concepts-placeholder15.C | 26 +++++++++++++++
> .../g++.dg/cpp2a/concepts-placeholder16.C | 33 +++++++++++++++++++
> 4 files changed, 94 insertions(+), 4 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-placeholder14.C
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-placeholder15.C
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-placeholder16.C
>
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 32640f8e9..2206d9ffe 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -31253,6 +31253,19 @@ do_auto_deduction (tree type, tree init, tree
> auto_node,
> full_targs = add_outermost_template_args (tmpl, full_targs);
> full_targs = add_to_template_args (full_targs, targs);
>
> + int want = TEMPLATE_TYPE_ORIG_LEVEL (auto_node);
> + int have = TMPL_ARGS_DEPTH (full_targs);
> +
> + if (want < have)
> + {
> + /* If a constrained auto is declared in an explicit specialization. */
> + gcc_assert (context == adc_variable_type || context == adc_return_type
> + || context == adc_decomp_type);
> + tree trimmed_full_args
> + = get_innermost_template_args (full_targs, want);
> + full_targs = trimmed_full_args;
> + }
> +
> /* HACK: Compensate for callers not always communicating all levels of
> outer template arguments by filling in the outermost missing levels
> with dummy levels before checking satisfaction. We'll still crash
> @@ -31260,11 +31273,10 @@ do_auto_deduction (tree type, tree init, tree
> auto_node,
> these missing levels, but this hack otherwise allows us to handle a
> large subset of possible constraints (including all non-dependent
> constraints). */
> - if (int missing_levels = (TEMPLATE_TYPE_ORIG_LEVEL (auto_node)
> - - TMPL_ARGS_DEPTH (full_targs)))
> + if (want > have)
> {
> - tree dummy_levels = make_tree_vec (missing_levels);
> - for (int i = 0; i < missing_levels; ++i)
> + tree dummy_levels = make_tree_vec (want - have);
> + for (int i = 0; i < want - have; ++i)
> TREE_VEC_ELT (dummy_levels, i) = make_tree_vec (0);
> full_targs = add_to_template_args (dummy_levels, full_targs);
> }
> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder14.C
> b/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder14.C
> new file mode 100644
> index 000000000..fcdbd7608
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder14.C
> @@ -0,0 +1,19 @@
> +// PR c++/114915
> +// { dg-do compile { target c++20 } }
> +
> +template<typename T>
> +concept C = __is_same(T, int);
> +
> +template<typename T>
> +void f() {
> +}
> +
> +template<>
> +void f<int>() {
> + C auto x = 1;
> +}
> +
> +int main() {
> + f<int>();
> + return 0;
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder15.C
> b/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder15.C
> new file mode 100644
> index 000000000..b507e4165
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder15.C
> @@ -0,0 +1,26 @@
> +// PR c++/114915
> +// { dg-do compile { target c++20 } }
> +
> +template<typename T, typename U>
> +concept C = __is_same(T, U);
> +
> +template<typename T>
> +int x = 0;
> +
> +template<>
> +C<double> auto x<double> = 1.0;
> +
> +template <typename T>
> +struct S {};
> +
> +template<typename T>
> +int y = 0;
> +
> +template<typename T>
> +C<char> auto y<S<T>> = 'c';
> +
> +int main() {
> + if (y<S<int>> != 'c')
> + return 1;
> + return 0;
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder16.C
> b/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder16.C
> new file mode 100644
> index 000000000..f808ef1b6
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder16.C
> @@ -0,0 +1,33 @@
> +// PR c++/114915
> +// { dg-do compile { target c++20 } }
> +
> +template<typename T, typename U>
> +concept C = __is_same(T, U);
> +
> +template<typename T>
> +struct A
> +{
> + template<typename U>
> + void f() {
> + }
> +};
> +
> +template<>
> +template<>
> +void A<int>::f<int>() {
> + C<int> auto x = 1;
> +}
> +
> +template<>
> +template<typename U>
> +void A<bool>::f() {
> + C<int> auto x = 1;
> +}
> +
> +int main() {
> + A<bool> a;
> + a.f<char>();
> + A<int> b;
> + b.f<int>();
> + return 0;
> +}
> --
> 2.45.2
>
>
